The field of the present invention relates generally to the use of ultra low calorific fuels and more particularly to methods and apparatus for using such fuels for energy production.
Ultra low calorific fuels, i.e., fuels having a heating value of about 40 MJ/kg or less, which may include furnace gases, biomass gasification, or fuels with CH4 content less than 10%, H2 content less than 10% and N2 content greater than 40% are typically “opportunity” fuels that are available as a waste stream or as the byproduct of a processing or manufacturing plant. Examples of these fuels are the so-called “blast furnace” gases (BFG) and “coke oven” gases (COG) that are generated from the smelting of iron. Even though such fuels are essentially “free,” there is a cost of compression for their use in a gas turbine. The combustion of ultra low calorific fuels in gas turbines generally present a significant design challenge given the large volumetric fuel flow required for a given energy input and the low reactivity of the fuel. To accommodate these fuels, at least some known combustion systems include fuel injectors that are large enough to pass the required site specific fuel volume such that air and fuel velocities are low enough for flame stabilization. Moreover, some known combustion systems rely on fuel blending with a more reactive fuel that enables the combustion of the gas in an existing low to medium calorific fuels combustor.
It would therefore be desirable to provide a combustor or combustion system that can utilize both ultra low calorific fuels and higher energy fuels, as desired, without having to rely on injectors that are designed to pass the entire fuel volume needed for each type of fuel, and to provide a combustion system that does not rely on fuel blending to burn ultra low calorific fuels.